In a conventional cathode ray tube (CRT) display such as used in oscilloscopes, a time varying electrical signal can be displayed on the CRT screen by sweeping the electron beam horizontally across the screen repetitively at a selected sweep rate while deflecting the beam vertically proportional to the magnitude of the electrical signal being monitored. If the sweep rate is properly timed with respect to the frequency of the signal, a relatively stable waveform will appear on the screen which can be examined by the operator. The conventional CRT analog oscilloscope can also be used to display signals which vary relatively slowly, using a relatively slow sweep of the electron beam across the face of the CRT. For such slowly varying signals, an entire waveform does not appear on the CRT screen but rather a moving "tail" or streak of light which corresponds to the leading portion of the waveform as the electron beam moves across the screen. Such relatively slow time varying waveforms are commonly encountered in medical monitoring equipment.
In modern, complex and computerized medical equipment, the CRT display often is the primary means of communicating information to the operator. Such equipment is also often utilized to perform substantial analysis on the input data signals and to display the results of such analyses to the operator along with the raw signal waveform information. Many such machines use a raster scanned CRT display in which the entire screen is scanned several times a second, allowing the simultaneous display of graphics, alphanumerics and waveforms. In raster scanned displays, the waveform does not change continuously as in analog oscilloscopes, but is refreshed at the raster scan frame rate, typically 60 frames a second.
One type of medical equipment in which raster scanned CRT displays have been used is electromyography (EMG) monitoring machines, in which a visual display is provided, in addition to an audio output signal, to allow examination of the electrical activity from a needle electrode inserted into the muscles of a subject under study. Skilled operators are adept at interpreting the simultaneous visual and auditory signals from electrical activity generated, either voluntarily or spontaneously, by the subject's muscles. The operator's movement of the needle electrode through the muscles also generates EMG activity. Prior to the introduction of raster scanned display EMG instruments, visual EMG records were produced using conventional analog oscilloscopes. Such conventional oscilloscopes are well suited to viewing real time, free running visual records such as those generated by EMG activity. Operators have attempted to use the newer digital raster scanned EMG instruments to produce real time, free running visual records of EMG activity as they had done previously with conventional analog oscilloscopes. However, the raster scan display of a waveform differs in significant respects from the conventional analog display. In particular, raster scan CRT screens ordinarily have a high resolution phosphor on the inside screen face which is characterized by low persistence, rather than the relatively high persistence phosphor used in conventional analog oscilloscope CRTs. The low persistence of the raster scan display screen insures that there is no interference in illumination between frames in the scan. However, because of the manner in which the real time waveforms appear on the raster scan display, EMG operators have noticed an illusion of a delay between the visual presentation of the waveform on the screen and the audio signal corresponding to the EMG activity, making it more difficult for the operators to correlate the audio and visual signals as they normally would.
Present raster scan display EMG monitors typically show a free running signal as a waveform which extends across the entire screen. Each time a new frame is displayed, the new data points acquired since the last frame are added to the leading edge of the waveform as new illuminated picture elements (pixels) and the pixels occupying the horizontal positions corresponding to the new data points are erased (non-illuminated). The pixels corresponding to the rest of the waveform remain illuminated and unchanged until the leading edge of the waveform passes them. A typical screen sweep width is approximately 100 milliseconds, and thus each of the illuminated points in a single sweep of the waveform will remain illuminated for a full 100 milliseconds. However, the period of visual latency is generally only about 40 milliseconds, and the time delay between the initial appearance of the waveform on the screen and its erasure may disrupt the operator's ability to coordinate the visual and auditory information, particularly during electrode insertion, since operators can perceive both the illumination of new data points and the erasure of old data points. The operators may thus find it difficult to properly coordinate the visual and audio signals.